1. Field of the Invention
The present invention relates to a charging control circuit for controlling an operation of charging a secondary battery of a cell phone, for example, and a charging device including the same. In particular, the invention relates to a charging control circuit and a charging device which reduce heat generated during charging.
2. Description of Related Art
General cell phones adopt a secondary battery (rechargeable battery) as a power supply. Referring to FIG. 7, a conventional charging device for charging a battery of a cell phone etc. is described. As shown in FIG. 7, a conventional charging device 101 includes a charging current supplying unit 110 for supplying a charging current to a secondary battery 111, and a charging control circuit 120 for controlling the charging current supplying unit 110. The charging current supplying unit 110 is composed of a charging transistor 112 connected with an adaptor voltage, a diode 114, a detection resistor 113, and the secondary battery 111, all of which are series-connected. A charging current I flowing through the charging transistor 112 is supplied to the secondary battery 111 to charge the secondary battery 111.
The charging control circuit 120 detects a potential difference (voltage) across the detection resistor 113 based on an amount of the charging current I supplied from the charging transistor 112 to the secondary battery 111, and controls the charging transistor 112 based on the detection result. That is, the charging control circuit 120 includes a detecting circuit 130 for detecting a voltage across the detection resistor 113, a control circuit 140 for outputting a control signal S1 in accordance with the detection result, and an operational amplifier 141 for controlling the charging transistor 112 in response to the control signal S1. The detecting circuit 130 includes an operational amplifier 131. The operational amplifier 131 has a negative terminal (inverting input terminal) connected with one end of the detection resistor 113 through an external contact 117 and a resistor 133. Further, a positive terminal (non-inverting input terminal) thereof is connected with the other end of the detection resistor 113 through an external contact 118 and a resistor 134. Furthermore, a feedback resistor 132 is connected between the negative terminal of the operational amplifier 131 and an output of the operational amplifier 131. The positive terminal thereof is connected with a grounded resistor 135. The charging control circuit 120 controls the charging transistor 112 to keep a voltage across the detection resistor at the same level.
Japanese Unexamined Patent Application Publication No. 9-84276 discloses a circuit for controlling an initial operation of charging such a secondary battery. A charging control circuit disclosed in this publication turns on a charging transistor for quickly charging the secondary battery in a short period, and after the battery was charged to the full, the circuit turns off the charging transistor and turns on a transistor smaller than the charging transistor. The smaller transistor is turned on to thereby continue a charging operation with a minute current not to overcharge the battery owing to the quick charging.
Incidentally, in recent years, devices using the secondary battery have been endowed with various functions. Along with such a tendency, there arises a need to increase a battery capacitance. To meet the need, an attempt has been made to increase a charging current for the purpose of shortening a charging period.
However, a larger amount of charging current causes a circuit supplied with the charging current to generate more heat. To overcome such a defect, some devices using the secondary battery have an adaptor (hereinafter, referred to as “current limiter-equipped adaptor”) having a current limiting function for preventing a current from flowing beyond a specified limit on a current value to charge a battery. The current limiter-equipped adaptor has characteristics as shown in FIG. 8. As shown in FIG. 8, the current limiter-equipped adaptor lowers an adaptor voltage if a current flows beyond the upper limit on the current value. Such an adaptor is used to carry out a general charging process, that is, constant current-constant voltage charging, thereby suppressing heat generation during a constant-current charging period in which a large among of charging current is supplied for charging.
FIGS. 9A and 9B are schematic diagrams showing current and voltage characteristics relative to a charging period. As shown in FIG. 9A, the charging period is generally divided into a pre-charging period T1, a constant-current charging period T2, and a constant-voltage charging period T3. During the pre-charging period T1, the secondary battery is charged with a constant current IS1 up to a predetermined voltage V1, for example, 3.2 V. During the constant-current charging period T2, the secondary battery is charged with a constant current IS2 larger than the current IS1 up to a predetermined voltage V2, for example, 4.2 V. During the constant-voltage charging period T3, after reaching the predetermined voltage V2, the secondary battery is charged with a charging current IS3 that is controlled to keep the voltage V2 at the same level.
In this case, the adaptor voltage changes as shown in FIG. 9B. Assuming that the adaptor voltage is 5.5 V, for example, during the pre-charging period T1, the battery is charged with a relatively small current IS1, so a voltage value Vadp thereof is kept at 5.5 V. During this period, the secondary battery is gradually charged up to the voltage V1. After the voltage level reached the voltage V1, the constant-current charging period T2 starts. During this constant-current charging period T2, if the charging current exceeds a predetermined current value, for example, as shown in FIG. 9B, the current limiter-equipped adaptor operates to lower the adaptor voltage Vadp.
Thus, during this period T2, an electric power represented by “(adaptor voltage Vadp−charging voltage Vb)×charging current” and applied to the charging transistor is lower than the case of the pre-charging period T1.
After the secondary battery was charged up to the voltage V2, the constant-voltage charging period T3 starts, and the charging current IS3 is controlled to gradually reduce for maintaining “a secondary battery voltage Vb=V2”.
If the charging current is increased to shorten a charging period, a generated heat quantity increases as mentioned above. As one measure therefore, the current limiter-equipped adaptor is used to suppress heat generation in this way. The applicants of the present invention have disclosed a charging control method and circuit capable of suppressing heat generation during a constant-current charging period, and a charging device including the same (Japanese Patent Application No. 2004-280845).
FIG. 9C is a schematic diagram showing a relation between a charging period and a current source temperature. If the secondary battery voltage reaches the predetermined voltage value V2, the charging control circuit starts charging (constant-voltage charging) with the secondary battery voltage kept at the voltage V2 (predetermined voltage). During the constant-voltage charging operation, the charging transistor is controlled to gradually reduce a charging current for charging the secondary battery. At this time, the connected adaptor increases its voltage value to the original value, 5.5 V, for example, in response to the reduction in charging current. Accordingly, a difference between the adaptor voltage Vadp and the charging voltage Vb of the secondary battery is largest at this point. That is, the electric power represented by “(adaptor voltage Vadp−charging voltage Vb)×charging current” and applied to the charging transistor reaches a peak just after the transition to the constant-voltage charging operation. An amount of heat generated from the charging transistor accordingly reaches a peak, and the charging transistor that is generally incorporated in a small package generates a large amount of heat. Therefore, it is desirable to suppress heat generation during not only the constant-current charging period but also the constant-voltage charging period.